We still have the crankcase bolts but if you damage or strip a head bolt you are in trouble as I do not have any spares. In a pinch the crankcase can be reworked to take M4 bolts but it also means work on the heads and is a lengthy procedure. The rocker cover bolts should either be a slot screw or will use a 2ba screw that undoes with a 1/16th allen key. Again if these screws are damaged there are no replacements but an M2 screw will fit in its place without modification. The carb and exhaust screws will be M3 and will work with either a 3/32 allen key (slightly sloppy fit) or 2.5mm.
All of these issues with mixed threads have now gone as all screws are metric and easy to obtain
There are ways to get the existing screws out of your engine if needed but the best advice I can give is not to take it apart at all. If its working well then it is best left alone!